The use of hydrogen gas (H2) fueled fuel cells such as polymer electrolyte membrane fuel cells (PEMFCs) offer the potential of reducing carbon dioxide (CO2) and eliminating nitric oxide emissions from vehicles However, current technology does not offer economically attractive options for storage of enough hydrogen gas to deliver the driving range to which motorists are accustomed. Instead of carrying a tank of hydrogen gas, vehicles could carry a tank of liquid fuel such as an alcohol. The alcohol, typically methanol, would pass through a fuel processor that converts the methanol to hydrogen gas that immediately passes to the fuel cell. In this fashion, hydrogen-powered vehicles need not carry any hydrogen tanks.
The process for converting methanol to hydrogen is known as “steam reforming” and is described by the following (unbalanced) chemical equation:CH3OH+H2O=CO+CO2+H2 To operate efficiently, the steam reforming reaction must be run in the presence of a catalyst. It has been reported by Isawa et al. that Pd/ZnO is a highly selective catalyst for steam reforming of methanol. See Catal. Lett. 19, 211-216 (1993).
The development of better steam reforming catalysts has long been an area of intense interest. An example of some recent research appears in published patent application EP 1 061 011 A1. In this publication, Wieland et al. report a supported PdZn/ZnO catalyst for methanol steam reforming. A catalyst (Example A) was made by wash coating gamma-alumina onto a ceramic honeycomb, impregnating the gamma-alumina with an aqueous solution containing Pd(NO3)2 and Zn(NO3)2, followed by drying, calcining at 500° C. and reducing at 400° C. The examples in this publication used a steam-to-carbon ratio of 1.5 and a liquid hourly space velocity (LHSV) of 5 h−1. Assuming a density of 0.96 g/ml for the feed, and assuming 100% conversion (note that the substantial increase in productivity from 300 to 350° C. indicates that conversion at 300° C. is substantially below 100% conversion), the maximum possible hydrogen productivity would have been 5500 ml H2/ml cat·hr.
For the purpose of developing an efficient fuel processor, weight and size of the energy device are major considerations. In order to reduce overall size of the on-board power system, insulating material should be minimized. This requires that steam reformer to be operated at relatively low temperature.